Tank gauging system and methods

ABSTRACT

Ultrasonic apparatus for measuring and remotely displaying the amount of liquid in a tank, the amount of water in the tank and the velocity of sound through the liquid in the tank is described with special application to a buried tank containing gasoline with the possibility of water in the bottom of the tank. The apparatus includes an ultrasonic transducer at a fixed distance above the bottom of the tank, a plurality of submerged reflectors vertically arranged at fixed distances above the transducer and a remote console, containing a computer, for activating the transducer and receiving signals of ultrasonic reflections developed by the transducer. Methods are disclosed utilizing the apparatus for computing the height of the liquid (gasoline) level, the depth of the water, if any, the velocity of sound through the gasoline and through the liquid as a whole, and for computing average temperature of the liquid in the tank. In the preferred embodiment, the reflectors are thin, planar sheets increasing in horizontal length with increasing distance from the transducer, the lower edges of which may be concave to maximize the reflections of ultrasonic pulses from the transducer back through the fluid to the transducer.

This application is a continuation of application Ser. No. 884,870,filed 7/10/86, which is a continuation of application Ser. No. 753,795,filed 7/8/85, which is a continuation of application Ser. No. 434,386,filed 10/14/82, the three last named applications all now beingabandoned.

BACKGROUND OF THE INVENTION

The present invention relates to the field of measuring the volume ofliquid in a tank and in particular, to making such measurement usingultrasound.

Many liquids, like gasoline, are stored in large underground tanks wherethe volume of the liquid in the tank cannot be observed directly. Theoldest and perhaps the most common way of determining the volume of suchliquid is to insert a calibrated rod into the tank and read the heightof the liquid from the line formed on the rod by the liquid's surface.

This method, however, provides only coarse estimates of liquid volumesbecause there are inherent errors in the system. For example, the personusing the rod for measurement might not insert the rod perfectlyvertically into the tank or might read the calibrations incorrectly.

The lack of precision provided in this method of volume measurement ismost limiting in attempts to detect the slow loss of gasoline due toleakage. Leakage of gasoline from underground tanks can cause seriousenvironmental problems and result in devastating legal liabilities. Ifsuch leakage could be detected at an early stage, the tank could berepaired or replaced at a fraction of that environmental and economiccost.

The use of a calibrated rod to measure the level of gasoline also hasthe added disadvantage of requiring the attendant to leave the securityof the service station building. This can be dangerous for the attendantand it leaves the building unattended and vulnerable to robbery orvandalism.

One development in the area of liquid volume measurements has beenultrasonic ranging systems. One such system uses ultrasonic transmitterspositioned above the liquid to measure the distance of the surface ofthe liquid from the transducer. Examples of this type of system areshown in U.S. Pat. No. 4,221,004 issued to Combs et al. on Sept. 2, 1980and U.S. Pat. No. 3,184,969 issued to Bolton on May 25, 1965. Theseultrasonic ranging systems are generally more accurate than a calibratedrod and they do not require a service station attendant to leave thebuilding.

Unfortunately, these types of ultrasonic ranging systems are still notaccurate enough for many uses. Such systems, for example, cannotdetermine whether a drop in the volume of the liquid is from loss of theliquid or from contraction due to a drop in the liquid's temperature.Adding a temperature detector will provide only limited correction sincethe temperature of gasoline usually varies over its volume and sincethere is no fixed coefficient of thermal expansion for gasoline which isnonhomogeneous and whose relative concentration of components varies.The knowledge of the temperature at one location in the gasoline is notvery helpful when temperature-compensating for volume measurements.

Another attempt to improve the accuracy of ultrasonic volume measuringsystems is the addition of calibrators. In U.S. Pat. No. 4,210,969issued to Massa on July 1, 1980, a small sound reflecting target locateda fixed distance from the surface of the ultrasonic transducer is usedto help to correct for variations in the velocity of sound in themedium. The system in Massa, however, only detects variations of sonicvelocity in the air above the liquid, and not variations in thegasoline.

The system described in U.S. Pat. No. 3,394,589 issued to Tomioka onJuly 30, 1968 is even more elaborate as it senses the reflections ofultrasonic energy off several equally spaced reflectors to make distancemeasurements. Although the additional reflectors can more accuratelycompensate for changes in sonic velocity through air, they also fail toaccount for changes of gasoline temperature.

Furthermore, ultrasonic ranging systems whose transducers sit above thegasoline cannot detect the presence of water in the bottom of the tank.If there is water below gasoline in a tank and a station operator,measuring the height of the liquid, does not recognize the presence ofthe water, the water may be accidentally pumped into a customer's tank.If this happens, the station operator runs the risk not only of losingcustomers, but also of possible legal action.

Conventional ultrasonic tank gauging systems whose receivers andtransmitters are positioned at the bottom of gasoline tanks fail tomeasure the amount of water in such tanks reliably. Examples of suchsystems are described in U.S. Pat. No. 3,693,445 issued to Johnson onSept. 26, 1972; U.S. Pat. No. 3,985,030 issued to Charlton on Oct. 12,1976; and U.S. Pat. No. 4,229,798 issued to Rosie et al., on Oct. 21,1970.

When the interface between the water in the gasoline lies very close tothe transmitter and the receiver, ultrasonic signals reflected off thegasoline-water interface interfere with the transmission of subsequentsignals and the tank gauge does not operate correctly.

The conventional tank gauging systems which have transducers beneath thesurface of the liquid do not correct for temperature changes of theliquid. Of the systems listed above, only Rosie et al. measures thetemperature of the liquid. Rosie et al., however, uses only atemperature sensor located at one position of the liquid.

One object of the present invention, therefore, is to measure the amountof liquid in a tank with great accuracy.

Another object of the invention is to detect very small leaks in a tankand the theft of small volumes of liquid.

It is also an object of the present invention to detect the presence ofwater in a gasoline tank and to measure the height of such wateraccurately.

Another object of this invention is a device and method for accuratelymeasuring the volume of gasoline in an underground tank while avoidingthe necessity of a service station employee's having to leave theservice station building.

Yet another object of the invention is to measure the amount of gasolinein a tank even when no service station attendant is on duty.

A further object of this invention is to facilitate automatic systemauditing so an inventory can be readily taken and product deliveriesrecorded, even when the service station is unattended.

Yet another object of this invention is to allow gasoline in a tank tobe monitored frequently with little error and to provide early warningalarms to a service station operator for any unusual conditions.

Additional objects and advantages of the present invention will be setforth in part in the description which follows and in part will beobvious from that description or may be learned by practice of theinvention. The objects and advantages of the invention may be realizedand obtained by the methods and apparatus particularly pointed out inthe appended claims.

SUMMARY OF THE INVENTION

To achieve the objects and in accordance with the purpose of theinvention, as embodied and as broadly described herein, the system ofthis invention for determining the amount of a liquid in a tankcomprises: a plurality of ultrasonic reflectors spaced at predetermieddistances from each other so that each of the reflectors lies at adifferent predetermined height in the tank; a transducer, positionedbelow the reflectors and designed to be located underneath the surfaceof the liquid in the tank and at a known height above the bottom of thetank, for transmitting ultrasonic energy into the liquid and for formingdata signals representing the reflections of the transmitted energy offof the plurality of reflectors and off of the surface of the liquid; andmeans for causing the transducer to transmit ultrasonic energy and forreceiving the data signals from the transducer in order to determine thevolume or height of the liquid in the tank.

More particularly, the system for determining the amount of liquid in atank comprises: a plurality of ultrasonic reflectors spaced atpredetermined distances from each other so that each reflector lies at adifferent predetermined height in the tank; a transducer positionedbelow the reflectors and designed to be located underneath the surfaceof the liquid in the tank and at a known height above the bottom of thetank, for transmitting ultrasonic energy into the liquid and for formingdata signals representing the reflections of the transmitted energy offof the plurality of reflectors and off of the surface of the liquid;means for measuring the temperature of the liquid in the tank; and meansfor causing the transducer to transmit ultrasonic energy and forreceiving temperature measurements from the temperature measuring meansand the data signals from the transducer in order to determine thevolume of the liquid in the tank.

The method of this invention for determining the amount of a liquid in atank of known dimensions comprises the steps of: transmitting, from atransducer located underneath the surface of the liquid, periodic burstsof pulses toward the surface of the liquid; measuring a surface echodelay time between the transmission of a first set of pulse bursts andthe receipt of the reflections of the first set of bursts from thesurface of the liquid; measuring a reflector echo delay time for each ofa plurality of submerged reflectors lying at different distances abovethe transducer, the reflector echo delay time for each reflector beingthe time between the transmission of a set of pulse bursts and thereceipt of the set of pulse bursts reflections from that reflector;calculating an average velocity of sound in the liquid from thereflector echo delay times and from the known distances of the pluralityof submerged transducers from the transducer; and calculating the amountof the liquid in the tank from the average sound speed and the surfaceecho delay time.

The method according to this invention of monitoring the volume of aliquid in a tank of known dimensions comprises the steps of: measuringthe volume of the liquid in the tank at periodic intervals during aperiod when no liquid is expected to be added to or removed from thetank; determining the temperature of the liquid during each volumemeasurement; normalizing the measured volume of the liquid to areference temperature; and comparing the volume measurements todetermine any decreases in the liquid volume.

The method of this invention for determining an average temperature of aliquid comprises the steps of: measuring the average sound velocity ofthe liquid in the tank; determining a reference sound velocity in theliquid, V_(R), at a reference temperature of the liquid, T_(R), and arelational correspondence between change in temperature and change insound velocity of the liquid ΔV/ΔT; and determining the averagetemperature of the liquid from v_(R),T_(R) and ΔV/ΔT.

A method of this invention for determining the amount of water in a tankcontaining liquid fuel and water comprises the steps of: transmitting,from a transducer located beneath the surface of the fuel and above thefuel-water interface, a periodic burst of ultrasonic pulses toward thegasoline surface; measuring the combined height of the fuel and water inthe tank using the pulse bursts; measuring the fuel-water interfacesecho delay time for a first set of pulse bursts to travel from thetransducer, reflect off the surface of the gasoline a first time,reflect off the fuel-water interface a second time and be received bythe transducer; and determining from said fuel-water interface echodelay time the height of the water.

Another method of measuring the amount of water in a tank containingfuel and water comprises the steps of: transmitting, from a transducerlocated beneath the surface of the fuel and above the fuel-waterinterface, a periodic burst of pulses toward the fuel surface; measuringthe temperature of the water and fuel in the tank; measuring, using thepulse bursts, the average velocity of sound in the fuel; measuring,using the pulse bursts, the average velocity of sound in the tank; anddetermining, from the measured temperature and from the differencesbetween the average velocity of sound in the fuel and the averagevelocity of sound in the tank, the height of water in the tank.

The accompanying drawings, which are incorporated and which constitute apart of this specification, illustrate one embodiment of the inventionand, together with the description, explain the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the tank gauging apparatus of thisinvention;

FIG. 2a shows a probe which could be used in the tank gauging apparatusof FIG. 1;

FIG. 2b is a side view of a reflector having a concave bottom edge;

FIG. 2c shows a detailed diagram of the console used in the tank gaugingapparatus of FIG. 1;

FIG. 3 shows typical electrical signals sent to and produced by thetransducer shown in FIG. 1;

FIG. 4 is a block diagram showing different methods of this invention;

FIG. 5 is a flow diagram for measuring the surface echo delay timeaccording to a method of this invention;

FIG. 6 is a flow diagram for determining the average velocity of soundin a liquid according to one method of this invention;

FIG. 7 is a flow diagram for measuring the closest reflector echo delaytime according to one method of this invention;

FIG. 8 is a flow diagram for measuring a next reflector echo delay timeaccording to one method of this invention;

FIG. 9 is a flow diagram for measuring an echo delay time in a windowaccording to one method of this invention;

FIG. 10 is a flow diagram for measuring a fuel-water interface echodelay time according to one method of this invention;

FIG. 11 is a flow diagram for measuring the average sound velocity in anentire tank according to one method of this invention;

FIG. 12 shows a typical transduced signal representing a reflectedultrasonic pulse;

FIG. 13 shows the path of an ultrasonic signal used to measure thefuel-water interface echo delay time according to the method shown inFIG. 10;

FIG. 14 shows the path of an ultrasonic signal used to measure thebottom echo delay time according to the method shown in FIG. 11;

FIG. 15 is a graph showing sound velocities in different liquids at thesame temperature;

FIG. 16a shows one embodiment of this invention for measuring thetemperature in a liquid;

FIG. 16b shows another embodiment for measuring the temperature in aliquid; and

FIG. 16c shows a third embodiment for measuring the temperature in aliquid.

DESCRIPTION OF THE INVENTION A. Tank Gauging System

An embodiment of the tank gauging system for measuring the amount ofliquid in a tank according to the the present invention is shown inFIG. 1. Tank 5 is filled with a liquid 50 whose volume or height is tobe measured by the tank gauging system of this invention. In thedescription that follows, the term "amount" is used to mean eithervolume or height.

Tank 5 is set below ground level and is physically accessible onlythrough manhole cover 2. The top of tank 5 includes standpipe 6 which isdirectly below manhole cover 2 and which is enclosed by standpipe cover8.

The tank gauging system in FIG. 1 includes a probe 7 extendingvertically into liquid 50. Probe 7 includes a plurality of verticallyaligned ultrasonic reflectors, shown as 10 in FIG. 1. In FIG. 1, theplurality of reflectors comprises individual reflectors 11-15 which eachlie at a different height within tank 5. The function of the reflectorsis explained more fully below but their general purpose is to reflectultrasonic energy and thereby assist in the calibration of the tankgauging system.

FIG. 2a shows probe 7 in more detail. The probe in FIG. 2a comprisesreflectors 105-113 which are held at their different heights byreflector support member 101. The tops of the reflectors may be beveledto reduce secondary reflections of ultrasonic waves by reflecting wavesstriking the top of the reflectors away from the probe and thetransducer.

FIGS. 2a shows a constant spacing between adjacent reflectors 10. Equaldisplacement of the reflectors is not required but, as will be apparentfrom the discussion below, the equal displacement of reflectors maysimplify certain steps in methods which use the tank gauging system ofthis invention.

The tank gauging system of this invention also includes a transducer 22positioned below the reflectors and designed to be located underneaththe surface of the liquid in the tank and at a known height above thebottom of the tank. Transducer 22 emits ultrasonic energy into theliquid and forms data signals from the received reflections of thatenergy off the bottom reflector edges 116 and off the surface of theliquid.

In the preferred embodiment of the tank gauging system of thisinvention, transducer 22 is a standard ultrasonic receiver/transmitterand forms a part of transducer assembly 20. It is also possible to havetransducer 22 contain a separate ultrasonic receiver and transmitter.The ultrasonic transducer chosen for the system of this invention mustbe able to receive the reflections of ultrasonic signals reflected offthe bottom edge 116 of each reflector and off the surface of the liquidin the tank.

FIG. 1 and FIG. 2a show that transducer 22 is supported at the top oftransducer assembly 20. Transducer assembly 20 is connected to thebottom of reflector support member 101 and provides a path for therouting of signals to and from transducer 22.

Probe extension 104, seen in FIG. 2a, extends from support 101 belowtransducer assembly 20 to prevent transducer assembly 20 from strikingthe bottom of the tank. Extension 104 can also maintain probe 7, andtherefore transducer 22, a known height above the bottom of the tank.

In the tank gauging system shown in FIGS. 1 and 2a, the reflectors mostdistant from transducer 22 are successively longer than the reflectorslocated closer to that transducer. This is done to ensure thatreflections from the bottom edges 116 of the more distant reflectors,which travel a longer path than do reflections from closer reflectors,are sufficiently large to be detected by transducer 22. All thereflectors in the tank gauging system of this invention can, however,have the same length. There are two principal limitations on the lengthsof the reflectors. First, the signals reflected from the most distantreflector must be able to be detected by the transducer 22 and second,the signals reflected from the reflectors must be discriminatablysmaller than signals reflected off the liquid surface.

The embodiment of the invention in FIG. 2a shows the reflectors 10 invertical alignment, but this is not required. Although it might seemthat with a vertical alignment of the reflectors the reflectors closerto the transducer would block the transmitted energy from the moredistant reflectors, this is not the case. Transmitted ultrasonic energywhich is not reflected by a reflector "bends" around that reflector andcan be reflected by more distant reflectors. Similarly, reflections ofultrasonic energy from a reflector "bend" around the reflectors lyingcloser to the transducer 22.

The surfaces of the reflectors facing the transducer can be concave asshown in FIG. 2b. To maximize the amount of ultrasonic energy reflectedback to the transducer 22, the radius of curvature of the concave edge116a of each reflector would equal the distance between the reflectorand the transducer 22. Such a design focuses the reflected ultrasonicsignal back to the transducer. Each reflector in such a system wouldhave a differently curved bottom surface.

A simpler approach to achieve the advantage of focusing is tomanufacture reflectors with concave bottom edges all having the sameradius of curvature. The most desirable curvature radius for suchreflectors would be the distance between the transducer and thereflector farthest from the transducer.

Instead of the continuously curved surface shown on the bottom 116a ofthe reflector in FIG. 2b the concave bottom surface may consist of anycurve or collection of segments that tend to focus incident ultrasonicsignals back to the transducer 22.

FIG. 3 shows typical electric signals at a transducer 22 according to anembodiment of the invention. Signal 301 is the electric signal sent totransducer 22. This is a negative signal which causes the transducercrystal to resonate and emit a burst of ultrasonic pulses into theliquid.

Signal 302 is formed by the transducer from the reflection of thetransmitted pulse off of the reflector closest to the transducer.Signals 303 and 304 are formed by the transducer from reflections of thetransmitted pulse off of more distant reflectors. Signal 305 is formedfrom the received reflection of a transmitted pulse off the surface ofthe liquid. Because the surface of the liquid is so large, the amplitudeof the ultrasonic reflections off the surface of the liquid issubstantially larger than the amplitude of the reflections of ultrasonicenergy off the reflectors.

Because the tank gauging system of this invention may be used withflammable liquids and liquids which will explode if ignited, it ispreferred that the tank gauging system be made explosion-proof orintrinsically safe.

In accordance with the present invention, the tank gauging system alsoincludes means for causing the transducer to transmit ultrasonic energyand for receiving data signals formed by the transducer in order todetermine the volume of the liquid in a tank. In the embodiment of thetank gauging system of this invention shown in FIG. 1, console 30,located inside service station building 40, is included in thetransmitting causing and signal receiving means.

A detailed diagram of the components in the preferred embodiment ofconsole 30 appears as FIG. 2c. Preferably, console 30 would include adata processor 200, for example a microprocessor, to give the tankgauging system of this invention flexibility and computing power. Dataprocessor 200 sends out control signals and receives data signals inorder to perform the desired processes.

One signal produced by data processor 200 is a trigger signal which issent to pulser 210. The trigger signal causes pulser 210 to transmit anactivation signal for tranducer 22. The activation signal, shown asnegative-going pulse 301 in FIG. 3, causes transducer 22 to transmit aburst of ultrasonic signals (typically about ten pulses at thetransducer's resonant frequency) into the liquid in the tank. Pulser 210could have a variable amplitude control which would be set by dataprocesser 200. In this manner, processer 200 could, by adjusting theamplitude of the activation signal, control the amplitude of thetransmitted ultrasonic signal.

Console 30 communicates with transducer 22 via coaxial cable 25 which iscarried inside conduit 26. As seen in FIG. 1, conduit 26 connects tostandpipe cover 8.

The reflections of these transmitted ultrasonic signals are converted bytransducer 22 into electrical signals (e.g., signals 302-305 in FIG. 3)and are sent to console 30 over coaxial cable 25.

In the preferred embodiment of the invention, console 30 also includessignal detection circuitry to convert the electrical data signals fromthe transducer into the form needed by data processor 200.

The signal detection circuitry should include means for adjusting itssensitivity. Such means may include, for example, a variable thresholdcircuit, a variable amplifier for increasing the amplitude of the datasignals sent from the trasducer, or circuitry for increasing theamplitude of the excitation signal sent to the transducer.

In console 30, the data signals from transducer 22 are amplified byamplifier 220 and wave-shaped by comparator 230. Comparator 230 issynchronized with the data signals so the comparator output changesstates when the data signal crosses the zero volt axis. Both the gain ofamplifier 220 and the threshold level (i.e. reference level) ofcomparator 230 are controllable by data processor 200 to adjust thesensitivity of the tank gauging system of this invention.

The output of comparator 230 indicates whether a signal of sufficientmagnitude is present. The comparator output is inputted to AND gate 240along with a "window" signal from data processor 200. The "window"signal is active during the time period in which data processor 200 issearching for a data signal from transducer 22. The output of AND gate240 indicates whether a transducer data signal of sufficient magnitudeis present during the time period represented by the "window" signal.

Timer 250 has a start and stop input and one data output. Timer 250 isstarted by data processor 200 when the trigger signal is sent to pulser210 and is stopped by the signal at the output of AND gate 240. Timer250 thus measures the time interval between the transmission of anactivation pulse and the receipt of a reflection from the that pulse(represented by a data signal from transducer 22) in the desired window.This time interval is available as an output to data processor 200.

Console 30 may also contain a display and control keys, identified inFIG. 2c by reference numeral 260. The program keys may be used to selectthe information displayed or to give commands and data to data processor200. The information to be displayed may include data, such as time,tank temperatures or liquid height and volume, or the display mayinclude instructions to the operator of the console.

Although FIG. 1 shows only one probe and one tank connected to console30, it is possible to have connected to console 30 several probes eachin a different tank. Such a system according to this invention wouldinclude means for causing the transducers in each of those probes totransmit ultrasonic signals and for receiving the data signals formed bythe transducers in each of those tanks in order to determine the volumeof liquid in each of those tanks. For example, such a system could havea console connected to several probes by several cables, each dedicatedto a single tank, or by a single cable which is time-shared by theprobes.

For certain measurements, some of which are described below, it isnecessary to know the temperature of the liquid or of a portion of theliquid in the tank. The tank gauging system of this invention can alsoinclude means for measuring the temperature of the liquid in the tank.In a preferred embodiment of this invention shown in FIG. 1, this meansincludes thermometer 60 which can be a standard temperature-sensingdevice that converts temperature measurements to electrical signals. Thetemperature measuring means is located in the liquid and is connected tothe transmitting causing and data receiving means (console 30 in FIG. 1)via the data communication lines. These data communication lines couldinclude coaxial cable 25 which connect transducer 22 to console 30 orcould include a separate set of coaxial cables.

This tank gauging system can measure the volume of liquid in a tank withgreat resolution, thus allowing detection of very small changes inliquid volumes and measurement of the amount of water in a gasolinetank. Furthermore all measurements with the system of this invention canbe made automatically, without the intervention of a service stationattendant. Several methods of measurements which can use the tankgauging system of this invention to achieve the objects of the presentinvention are explained below.

B. Measurement of Average Sound Velocity for the Liquid in a Tank

FIG. 4 shows a block diagram indicating different measurements ordeterminations which can be made with the tank gauging system of thisinvention. All of these measurements require the measurement of theaverage sound velocity for the liquid in a tank. Blocks 401 and 402 showthe major steps for making such a measurement very accurately.

1. Measurement of surface echo delay time

The first step in such a measurement, step 401, is to measure a surfaceecho delay time, referred to as ET_(s). ET_(s) is the time it takes foran ultrasonic signal to be transmitted from a transducer, to bereflected off the surface of the liquid in the tank and be received backby the transducer. The detailed steps of this measurement are shown inFIG. 5.

The sensitivity setting and adjustment of the signal detection circuitryis described below in terms of threshold. It should be recognized thatthe sensitivity setting could also be, for example, the amplificationfactor (gain) of received data signals from a transducer or theamplification factor (gain) of the activation signal sent to atransducer.

In step 501, the threshold of a receiving device, for example the signaldetection circuitry in console 30 of FIG. 2c, is set at a predeterminedvalue. This predetermined value, if the sensitivity control is athreshold, can either be the maximum threshold setting of the receivingdevice or some predetermined level known to be appropriate for detectingsurface reflections.

In the next step, step 502, a determination is made whether an echo,i.e. a reflection of an ultrasonic signal, has been received. Todescribe this step in further detail, reference is made to FIG. 12 whichshows a signal representing the reflection of an ultrasonic signal offthe surface of the liquid. When the threshold of the receiving device isset at the level x, for instance, the transmitted signal's reflectionshown in FIG. 12 will not be detected.

If no echo or reflection is received, the threshold of the receivingdevice is decreased, step 503, and another inquiry is made regarding thereceipt of an echo, step 502. The loop comprising steps 502 and 503 iscontinued until an echo is received. Referring to FIG. 12, when thethreshold is decreased to value y, then the receiving device will detectthe presence of the illustrated reflection. If the console in FIG. 2c isused with this method processor 200 adjusts the sensitivity anddetermines whether an echo was received.

Because echoes from the surface are, in this method, detected beforeechoes from the reflectors, the reflectors must be designed to ensurethat the amplitude of their echoes is sufficiently smaller than theamplitude of the surface echoes to allow discrimination between surfaceechoes and reflector echoes.

When an echo is received, it must be determined whether this was thefirst echo received, because a slightly different procedure is followedfor first echoes. This determination is step 510.

If the most recently received echo was the first echo received, then theecho delay time, which is the time between the transmission of anultrasonic pulse and the measured receipt of the echo, is recorded asthe new value for ET_(s). The echo delay time appears at the output oftimer 250 in the embodiment shown in FIG. 2c.

If the threshold were set at value y, then the time of receipt of thatpulse would be at time y'. In using the preferred embodiment of thecircuitry shown in FIG. 2c, the time which would be recorded is y", thefollowing zero-crossing. The recorded receipt times of different pulsesin a burst thus differ by integral numbers of pulse periods.

After the new value for ET_(s) is recorded, the threshold is againdecreased in step 512 in an attemp to detect an earlier one of thereflected pulses in a burst. The most accurate measurement of ET_(s) isobtained by detecting the reflection of the first-transmitted pulse inthe burst.

If, upon decreasing the threshold, predetermined minimum threshold isreached, then the measuremet of ET_(s) is complete and is the mostrecently measured value for ET_(s). The determination of a minimumthreshold indicates that no more accurate measurement of ET_(s) ispossible.

If the threshold is not at a minimu, then the next echo is received instep 515 and the same question is asked with regard to whether this echowas the first received. If this echo is not the first received, then acomparison is made between the pulse period (one full cycle of a pulsein the transmitted burst) and the difference between the last measuredecho delay time and the previously measured echo delay time.

If, for example, the threshold were set at level w, then the echo delaytime would be measured from point y" (not w' for the reason describedabove). In this case, the last two echo delay times would be equal andtheir difference would be less than one pulse period so steps 512 and514 would be repeated. Assuming that the threshold was not at a minimum,another echo would be received with the receiver set a lower treshold.

If the next threshold value were level z in FIG. 12, then the echo delaytime would be measured from point z". The difference between the lasttwo measured echo delay times, then, would be equal to one period. Inthis case, the echo delay time measured from point z" would then be thenew value for ET_(s) and steps 512 and 514 would be repeated.

This method of "stepping back down" the received ultrasonic pulse burstis continued until either a minimum threshold is reached, step 514, orthe third condition in step 520 is encountered: the difference betweenthe last two echo delay times is greater than one pulse period. Thisthird condition occurs if the last received echo signal is noise ratherthan a reflection of the transmitted signal. In such a situation, nomore accurate a value for ET_(s) can be determined so the echo delaytime for the signal received prior to receiving the noise signal isdeemed to be the most accurate value for ET_(s).

By making the period of the transmitted pulse, which is a function ofthe resonant frequency at the transducer crystal, sufficiently short,any error in measuring ET_(s) can be maintained at a low value.Furthermore, by stepping slowly back down the different pulses of thereceved pulse burst, it can be determined that the received signal isactually a reflection rather than a noise signal.

In measuring the surface echo delay time, disturbances on the surfacemay cause some variation in the amplitude of the echoes or reflectionsreceived by transducer 22. To reduce the possibility that thesevariations will cause an erroneous reading of ET_(s), the threshold canbe set at a value intermediate between the value needed to detect theecho being received and the value needed to detect the next earlier echopulse in the burst.

Additional accuracy can be gained by taking multiple measurements ateach threshold setting and averaging the measurements.

2. Measurement of reflector echo delay times

Returning to FIG. 4, the step of measuring the average sound velocity,step 402, requires measuring reflector echo delay times, the steps forwhich are shown in FIGS. 6-9.

In FIG. 6, the first step, 601, is to measure the closest reflector echodelay time referred to as ET_(c). In this method, there are a pluralityof reflectors, as shown, for example, in FIGS. 1 and 2a.

The details of the measurement for ET_(c) are shown in FIG. 7. The firststep of such a measurement, step 701, involves the setting of a windowfor the closest reflector. That window is a time window in which to lookfor the ultrasonic pulse reflections from the reflector closest to thetransducer. The duration and timing of this window is determinable from,for example, the distance between the transducer and the reflectorclosest to the transducer and from the possible range of soundvelocities in the liquid being measured. The value for this windowwould, in the preferred embodiment of the tank gauging system describedabove, be stored in the data processor.

Once this window is set, ET_(c) is measured from the reflectionsreceived in that window. The method of making this measuement is shownin FIG. 9, which method is similar to that outlined in FIG. 5. Since thereflected signals from the reflectors will be smaller in amplitude thanthe signals reflected from the liquid surface, the threshold of thereceiver in the preferred embodiment of this method is initially set toa lower value than it is when receiving reflections from the surfae ofthe liquid. In FIG. 9, the threshold is set (step 901) and is decreased(steps 902 and 903) until an echo is received (step 902).

When the first echo is received (step 910) the value for the mostrecently measured echo delay time (actually the zero-crossing time) isrecorded as the new value for the echo delay time for the particularreflector (step 911). The threshold is then decreased (step 912) and, ifthe threshold is at a minimum (step 914), then the last-measured echodelay time is the value for the reflector echo delay time. If thethreshold is not at a minimum, then the next echo is received and theperiod of the transmitted or received pulse is copared to the differencebetween the last two measured echo delayed times (step 920). If thatdifference is less than one pulse period, then steps 912 and 914 arerepeated.

If that difference is equal to one pulse period then the echo delay timefrom the most recently received echo becomes the new reflector echodelay time (step 911) and the threshold is decreased again (step 912).If, however, the difference between the last two measured echo delaytimes is greater than one pulse period, then it is assumed that the mostrecently received echo was a noise signal and the reflector echo delaytime is the last value entered for that time.

Once the closest reflector echo delay time is determined in step 601 ofFIG. 6, it must be determined whether there are additional submergedreflectors. This can be done, for example, from the heights of all thereflectors and ET_(s). If there are additional submerged reflectors,then step 611, which requires the measurement of the echo delay time forthe next most distant reflector from the transducer, is performed. Thedetails of measuring the echo delay time for next reflector at the delaytime are shown in FIG. 8.

In step 801, a window for the next reflector is set based on themeasured and estimated echo delay times for the last reflector, i.e.,the reflector located next closest to the transducer. For eachreflector, there is stored an estimated echo delay time similar to theestimated window for the closest reflector. After the echo delay timefor a reflector is measured, a proportionality constant is calculated asthe ratio of the measured echo delay time to the estimated echo delaytime of that reflector. This proportionality constant indicates, in ageneral manner, how much the measured echo delay time for that reflectordiffers from the estimated echo delay time and provides a tool forestimating how much the next reflector's measured echo delay time willdiffer from its estimated echo delay time.

The proportionality constant is multiplied by the estimated echo delaytime for the next reflecor to form a revised estimated echo delay timefor the next reflector. The window in which reflections from the nextreflector will be searched is centered around this revised estimate forthe next reflector echo delay time.

In step 802, an echo delay time for the next reflector is measured fromreflections received in the window for that reflector. The details ofthis measuremet are seen in the method shown in FIG. 9. The individualsteps of that method have been previously explained.

3. Determination of average sound velocity

If there are no other submerge reflectors, the average sound velocity inthe liquid, v_(L), can be-calculated from the known height of thehighest submerged reflector, H_(HR), and from the echo delay time of thehighests submerged reflector ET_(HR). Thus:

    v.sub.L =(H.sub.HR -H.sub.T)(2/ET.sub.HR). H.sub.T is the height of the transducer above the bottom of the tank.

The reason for making this determination by the method outlined in FIG.6 is to provide great accuracy for the determination of the soundvelocity in the liquid. Since certain liquids, like gasoline, arenonhomogoneous, the sound velocity may vary slightly in differentportions of the liquid volume. The most accurate determination of soundvelocity from this method requires measurements which take into accountthe maximum liquid volume, hence the submerged reflector closest to thesurface is used. It is difficult, however, to detect which reflectionsare from the highest submerged reflector. That is why reflections fromthe closest reflector, to the transducer are detected first andreflections from successively farther transducers are detected until thereflections from the highest submerged reflector are found.

C. Measurement of Height or Net Volume of the Liquid in a Tank

The measurement of net volume includes steps 403-412. In step 403, theheight of the liquid above the bottom of the tank, H_(L), is determinedfrom the surface echo delay time, ET_(s), and from the velocity of soundin the liquid, v_(L), by the following equation:

    H.sub.L =v.sub.L (ET.sub.s /2)+H.sub.T

Once the height of the liquid about the bottom of the tank is known, thegross volume of the liquid (see step 412), V_(L), can be determinedeither mathematically, if the exact geometry of the tank is known, orfrom a table relating the height of liquid in the tank to its volume.

The accuracy of this determination of gross volume comes from theprecise measurements of ET_(s) and v_(L) afforded by the method of thisinvention. Such accuracy allows this invention to accomplish itsobjects.

This determination of the gross volume may not be accurate enough formany applications because the volume of a liquid varies with thetemperature of that liquid and the variation in a particular liquid maymask a drop in volume due, for example, to leakage. This variation withtemperature is usually by a value called the Temperature ExpansionCoefficient or TEC.

In step 411, the gross volume is temperature-corrected by multiplying itby (1+TEC(T_(R) -T_(AV))). TEC is the temperature expansion coefficientfor the particular liquid, T_(R) is a reference temperature and T_(AV)is an average temperature of the liquid. A method for accuratelydetermining the average temperature of the liquid is described below.

The gross volume of liquid, however, may not be of interest, especiallyif the tank contains gasoline. In gasoline tanks, there is often anaccumulation of water at the bottom of the tank. To determine the netvolume of gasoline in a tank, it is necessary to subtract from the grossvolume of liquid in the tank V_(L), the volume of water, V_(W), in thetank (step 414). There are described below techniques for determiningthe volume of water in a tank.

The accuracy of the method of determining the net volume of liquid in atank just described makes it ideal for use in detecting the loss ofproduct due to theft or leakage.

D. Theft or Leak Detection

Of course, one method of detecting the theft of liquid from a tank is bynoting unauthorized dispensing of that liquid. There may be instances,however, when such a method will not work. Furthermore, it is may bedesired to detect leakage of the liquid from the tank. Leakage occurs ata very slow rate so accurate determinations of the volume of the liquidin the tank are needed for such detection.

A method of theft or leak detection using the gross volume measurementmethod described above is shown in FIG. 4, boxes 420-422. The method canalso compae net volumes, however it is important to know whether theleakage from the tank involves only, water and a comparison of netvolume will only identify a leakage of fuel.

In step 420, there is a comparison of temperature corrected grossvolumes of liquid during periods of no activity. A period of no activityoccurs when no deliveries are made to the tank of liquid and noauthorized dispensing of that liquid is taking place. In a servicestation, this can occur overnight or on holidays, or such a period canbe regularly scheduled so that a particular tank is shut down to checkfor leakage or theft.

Because the volumes compared are temperature-corrected, this method candetermine whether any decrease in volume is due to a change intemperature or due to the loss of liquid by theft or by leakage.

In step 421, any decreases in temperature-corrected volumes are noted,and in step 422, the rate of such decreases are determined from themeasured loss in volume and the time during which that loss occured.From the rate of decrease, it can be determined whether a loss in volumeis signficant, and, if it is, whether that loss is due to theft orleakage. For thefts the rate of change of volume will not be steady:large losses of volume over a very short time and no changes in volumefor long periods of time. In the case of leakage, there will be small,relatively constant decreases in volume over very large periods of time.

In leakage detection especially, it is necessary to have a highresolution volume measurement, and the method described above formeasuring the volume of liquid in a tank for a range of temperatures isideally suited for such detection.

E. Water Volume Measurement Method I

One method of water volume measurement according to the presentinvention is shown in steps 401-403 and 430-32. First, the surface echodelay time, ET_(s), the average sound velocity of the total amount ofliquid in a tank, v_(L), and the total height of the liquid above thebottom of the tank, H_(L), are all determined as indicated above (steps401-403). The next step is to measure the fuel-water interface echodelay time, ET_(FW), a indicated in step 430. ET_(FW) differs from theother echo delay times described since its measurement involves threereflections as seen by reference in FIG. 13.

As FIG. 13 shows, ET_(FW) has four components comprising the transmittedsignal and three reflections. The first component is time t₁, which isthe time it takes an ultrasonic pulse to travel from the transducer tothe surface of the liquid in the tank. The second component of ET_(FW)is t₂, which is the time it takes for the ultrasonic pulse to travelbetween the surface of the liquid of a tank and the fuel-water interfaceafter the ultrasonic pulse has been reflected the first time. The thirdcomponent of ET_(FW), t₃, is the time it takes the ultrasonic pulseburst to travel between the fuel-water interface and the surface of thefuel after being reflected off the fuel-water interface. The finalcomponent of ET_(FW) is shown in FIG. 13 as t₄ which is the time ittakes the ultrasonic pulse burst to travel between the surface of theliquid and the transducer after being reflected a third time.

The details for measuring the fuel-water interface echo delay time areshown in FIG. 10. First, as step 1001 indicates, a window is estimatedfor receiving the reflections of the fuel-water interface. This estimatecan be based upon the determine of values for ET_(s), v_(L) and H_(L),as well as on an estimated range of water level heights.

The next step, step 1002, is to look for reflections in the estimatedwindow and measure ET_(FW). The measurement of ET_(FW) involves themethod shown in FIG. 9 which has been previously explained.

Once ET_(FW) is measured, then the height of the water in the tank,H_(W), is determined according to the equation shown in step 431:

    HW=v.sub.L (ET.sub.s -ET.sub.FW /2)+H.sub.T.

The derivation of this equation stems from the definition of ET_(FW). Asseen in FIG. 13: ##EQU1## where ET_(W) is twice the time required for anultrasonic pulse to travel between the surface of the liquid and thefuel-water interface.

Thus:

    ET.sub.W =ET.sub.FW -ET.sub.s.

The height of the water, H_(W) is equal to the difference between theheight of the liquid, H_(L), and the distance between the surface of theliquid and the fuel-water interface. In other words:

    H.sub.W =H.sub.L -v.sub.L (ET.sub.W /2). From the equations for H.sub.L (step 403) and ET.sub.W : ##EQU2## This last equation is the equation set forth in step 431 in FIG. 4.

In step 432, the volume of water, V_(W) is determined from the height ofthe water, H_(W), in much the same way as the gross volume of theliquid, V_(L), is determined from the height of the liquid, H_(L), instep 410. This can be either by a table lookup method or by calculationif the geometry of the tank is known. Usually, water height rather thanwater volume is displayed since the nearness of the water to thetransducer is of primary importance.

This method for measuring the volume of water is especially effective ifthe fuel-water interface is not close to the level of the transducer. Ifthe transducer is close to the fuel-water interface, then thereflections from the fuel-water interface may create problems indetecting ET_(FW).

Typically water height rather than water volume will be displayed to theoperator since water height gives a more readily observed indication ofwhether the amount of water in the tank is at a dangerous level.

Method II

Another method of measuring water volume according to this invention isseen in FIG. 4 by the steps 401-403 and 441-443 and 432.

In step 441, the temperature of the liquid in the tank is measuredeither by a thermometer or by some other method, such as the methoddescribed below. In step 442, the average sound velocity in the entiretank, v_(T), is measured. This measurement differs from the measurementin step 40 which only measured the velocity of sound through the liquidabove the transducer. If there was water at the bottom of the tank, thesound velocity that was measured in step 402 is just the sound velocitythrough the fuel and not through both the fuel and the water.

The detailed steps for measuring the average sound velocity in theentire tank are shown in FIG. 1. In the first step of that method ofmeasurement, a window for receiving reflections off the bottom of thetank (or off a lip on the transducer which lies below the fuel waterinterface) is estimated. Such an estimate may be made from ET_(s), v_(L)and H_(L).

The echo delay time being measured in FIG. 11 is the echo delay time offthe bottom of the tank, ET_(b), as shown in FIG. 14. Rather than usingreflections of the bottom of the tank, ET_(b) can be measured usingreflections off a transducer lip. The lip on the transducer is usedespecially when the bottom of the tank is believed to contain debris.

ET_(b) is equal to the sum of times t₅ -t₈. Time t₅ is the time it takesan ultrasonic pulse to travel from the transducer to the surface of thefuel. The time for the pulse to travel from the fuel surface to thebottom of the tank or the transducer lip after the first reflection istie t₆, and the time for that reflection to travel back from the bottomof the tank or transducer lip to the surface of the fuel is time t₇.Time t₈ is the time it takes for the ultrasonic pulse to travel from thesurface of the fuel back to the transducer after being reflected a thirdtime.

Next, ET_(b) is measured by detecting reflections in the estimatedwindow for the bottom echo delay time (step 1102). Again, in thepreferred embodiment this measurement uses the method shown in FIG. 9.

Step 1103 (see FIG. 11) contains an equation for determining the averagevelocity for the entire tank. The average sound velocity of the entiretank, v_(T), is the distance from the surface of the liquid to thebottom of the tank, H_(L) +H_(T), divded by the time it takes for anultrasonic pulse to travel that distance, t_(B). If the reflections offthe transducer lip are used instead, the value for H_(T) is adjustedaccordingly.

FIG. 14 shows that: ##EQU3##

Thus:

    t.sub.B =(ET.sub.B -ET.sub.s)/2; and

    v.sub.T =(H.sub.L +H.sub.T)2/(ET.sub.b -ET.sub.s)

The height of the water in the tank is determined in step 443 bycomparing the average sound velocities in the fuel, v_(L), and in theentire tank v_(T), for the measured temperature. If the velocity ofsound is sufficiently different through water and through fuel for themeasured temperature, graphs, such as the one in FIG. 15 which shows arelationship between velocity of sound through a fuel and through waterat a certain temperature, can be used to determine what height of watermust be contained in the tank to account for the differences in soundvelocities in the entire tank and in the fuel.

Once the height of the water is known, the volume of the water can bedetermined by methods described in relationship to step 432.

This method of measuring water volume is most accurate when the volumeof water is substantial and thus can be used in those instances when thefuel-water inerface is close to the height of the transducer. Thismethod works best when the velocity of sound through fuel is appreciablydifferent at all temperatures from the velocity of sound through water.

Another advantage of this method is that the change in the velocity ofsound with temperature throuhg many fuels is in a direction differentfrom the sound change in the velocity of sound with temperature throughwater.

F. Determination of Average Temperature

A method of determining the average temperature of the liquid in thetank is shown in FIG. 4 by steps 401, 402 and 453. Additionally, steps451 and 452 can be used to provide greater accuracy for that temperaturemesurement.

The surface echo delay time, ET_(s), and the average sound velocity ofthe liquid in the tank, v_(L), are determined in steps 401 and 402.

The average temperature of the liquid can be determined in step 453 ifseveral values are known. The first value that must be known is how thevelocity of sound in that liquid changes with the change in temperature,Δv/ΔT. This value has been determined for many liquids. The other valuethat must be known is the velocity of sound through the liquid, V_(Ref),at some reference temperature, T_(Ref).

The change in velocity of the sound through the liquid, v_(L) -v_(Ref),divided by the difference between the liquid's temperature and thereference temperature, T_(Av) -T_(Ref), should equal Δv/ΔT. In otherwords:

    T.sub.Av =(v.sub.L -v.sub.Ref)/(Δv/ΔT)+T.sub.Ref.

This equation can be simplified T_(Ref) is made equal 0 degrees.

If the values for Δv/ΔT and v_(Ref) at T_(Ref) are not known, or if itis desired to know value with a greater accuracy, these values can bedetermined by the steps 451 and 452.

In step 451, the velocity of sound through a portion of the liquid ismeasured periodically and for each such velocity measurement, atemperature measurement is also taken. Such a temperature measurementcan be taken by some type of thermometer. In a preferred embodiment ofthis method, the sound velocity and temperature would be measured in theregion between the transducer and the reflector lying closest to thetransducer. The advantage of taking the measurements in this region isthat in a small region, the temperature is relatively constant.

The sound velocity can be measured by determining ET_(c), the echo delaytime of the closest reflector, and dividing it into twice the distancebetween the transducer and the reflector closest to the transducer.

FIGS. 16a-16c show three possible placements for the thermometer. Such adevice can be located adjacent to the closest reflector, shown in FIG.16a, at a point between the closest reflector and the transducer, shownin FIG. 16b, or adjacent the transducer, shown in FIG. 16c.

When several velocity and temperature measurements have been taken, Δvand ΔT can be determined accurately for the liquid at particulartemperatures. In addtion, the reference velocity of the liquid, v_(Ref),can also be determined for a reference temperature, T_(Ref), with greataccuracy, hence the average temperature determination in step 453 can bevery precise.

It will be apparent to those skilled in the art that modifications andvariations can be made in the tank gauging system and methods of thisinvention. The invention in its broader aspects is not limited to thespecific details in the representative methods and preferred embodimentsshown and described. Departure may be made from such details withoutdeparting from the scope an spirit of the general inventive concept.

What is claimed:
 1. A system for determining the amount of liquid in atank comprising:(a) support member; (b) a plurality of ultrasonicreflectors spaced at predetermied distances from each other along saidsupport member, so that each of said reflectors lies at a predeterminedheight above the bottom of said tank; (c) transducer means positionedbelow said reflectors and located underneath the surface of and withinsaid liquid in said tank and at a known height spaced from and above thebottom of said tank, said transducer means being operable fortransmitting ultrasonic energy upwardly through said liquid towards theunderside of said surface and for forming data signals representing thereflections of said transmitted energy off of the submerged ones of saidplurality of reflectors and off of the surface of said liquid, and (d)control means for operating said transducer means, said control meansincluding signal processing means for receiving said data signals fromsaid transducer means, signal detection circuitry coupled to said signalprocessing means and including means for varying detection sensitivityin a predetermined sequence of successive steps of increasingsensitivity until said data siganls representing reflection of saidtransmitted energy off of said surface of said liquid are received anddetected, means for computing an average velocity of said ultrasonicenergy in said liquid from said detected data signals representative ofreflections from the highest one of said submerged reflectors, forcomputing the height of said surface of said liquid from said bottom ofsaid tank utilizing said computed average velocity and for therebydetermining the amount of liquid in said tank.
 2. The system in claim 1wherein said ultrasonic reflectors are vertically aligned.
 3. The systemin claim 2 wherein said ultrasonic reflectors are equally spaced fromone another.
 4. The system in any one of claims 1, 2 or 3 wherein themore distance an ultrasonic reflector is from said transducer, thelonger is its horizontal dimension.
 5. The system in claim 1 furtherincluding a substantially vertical support member connected to saidreflectors and to said transducer.
 6. The system in claim 5 wherein thebottom surfaces of said ultrasonic reflectors are concave.
 7. The systemin claim 6 wherein the radius of curvature of each of said concavesurfaces of said reflectors is constant.
 8. The system of claim 1wherein said signal detection circuitry includes means for varying athreshold, at which signal detection occurs in successive steps ofdecreasing threshold until data signals representing reflection of saidtransmitted energy off of said under surface of said liquid are receivedand detected.
 9. The system in claim 1 wherein said signal processingmeans includes a microprocessor.
 10. The system in claim 1 wherein saidcontrol means further includes:(i) a pulser connected to said signalprocessing means for causing said transducer means to transmitultrasonic energy into said liquid; (ii) signal detection circuitry forreceiving said data signals from said transducer means and forindicating whether a reflection of said transmitted energy was receivedby said transducer means within a predetermined time window; and (iii)timer means connected to said signal processing means and said signaldetection circuitry to measure the time between the transmission ofultrasonic energy and the receipt within said predetermined time windowof reflections of said transmitted energy.
 11. The system in claim 10wherein said transducer means is responsive to pulses from said pulserfor transmitting ultrasonic energy of variable amplitude.
 12. A systemfor determining the liquid in a tank comprising:(a) a plurality ofultrasonic reflectors spaced at predetermined distances from each otherso that each of said reflectors lies at a different predetermined heightin said tank; (b) a transducer, positioned below said reflectors anddesigned to be located underneath the surface of said liquid in saidtank and at a known height spaced from and above the bottom of saidtank, for transmitting ultrasonic energy through said liquid and forforming data signals representing the reflections of said transmittedenergy off of the submerged ones of said plurality of reflectors withinsaid liquid, and off of the under surface of said liquid; (c) controlmeans for causing said transducer to transmit ultrasonic energy, saidcontrol means including signal processing means for receiving said datasignals formed by said transducer, signal detection circuitry coupled tosaid signal processing means and including means for varying detectionsensitivity in a predetermined sequence of successive steps ofincreasing sensitivity until said data signals representing reflectionof said transmitted energy off of said surface of said liquid arereceived and detected, means for computing an average velocity of saidultrasonic energy in said liquid from said detected data signalsrepresentative of reflections from the highest one of said submergedreflectors; and (d) means for calcualting a temperature of said liquidin said tank from a liquid reference temperature and said averagevelocity as computed from said data signals in order top accuratelydetermine the temperature compensated volume of said liquid in saidtank.